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With a lot of rain, wind and even recent snow on the mountains, Lake Te Anau was probably around 10 degrees Celsius, Mr Inder said.

Queenstown Lakes harbourmaster Marty Black said with Lake Wakatipu at 10C or 11C, and Lake Wanaka 14C to 15C, users should be prepared.

A wetsuit or a life jacket, while helping buoyancy, could also combat hypothermia, he said.

The reminder came after Invercargill's Brent Hewitt rescued a girl in Lake Te Anau during the weekend. He entered the lake and swam to the aid of the girl struggling in the cold water. "It's just one of those things you do," Mr Hewitt said.

He was riding his bike along the lake front when he saw the girl going under the water, he said. He managed to help her on to a pontoon.

He said it was a shock how cold the water was, and the temperature almost got the better of him.

"The pontoon is only spitting distance from the shore but the cold hits you," he said.

Mr Hewitt said he did not feel like a hero, deflecting much of the praise to another girl in the water.

"She also noticed the other girl in trouble and reacted by swimming back and keeping the head of the girl in trouble above the water.

"That girl deserves most of the credit for saving a life."

He said he was a confident swimmer but the cold knocked him about.

"I would encourage everyone to be confident in the water, but also a fit swimmer. I will be going to the pool to do some more training," he said.

"It would be the worst feeling in the world not being able to help or going to help but then needing to be saved yourself."

Mr Hewitt ensured everyone made it safely ashore before slipping off with his wife for a hot cup of tea.